Lithium-ion secondary batteries have a higher energy density compared to other secondary batteries such as nickel-cadmium batteries, nickel-hydrogen batteries, or lead storage batteries. Thus, lithium-ion secondary batteries are used as power sources for portable electronic devices such as notebook computers and mobile phones.
Recent trends in development of lithium-ion secondary batteries include downsizing of batteries for resource saving and cost reduction, as well as expansion of applications to electric vehicles and power sources for electricity storage. Thus, there is a need to increase the density of the negative electrode for achieving a higher capacity, a higher input-output efficiency, and cost reduction. High-crystallinity carbon materials, such as artificial graphite or spherical natural graphite obtained by spheroidizing flake-shaped natural graphite, have drawn attention as a material for obtaining a high-density negative electrode.
In the case of artificial graphite, as described in Japanese Patent Application Laid-Open (JP-A) No. H10-158005, cycling performance and rapid charge-discharge properties are improved by using a graphite particle, having a secondary particle structure in which plural primary particles having a compressed shape aggregate or are combined so as to have nonparallel orientation planes, for a negative electrode material.
In a lithium-ion secondary battery, an energy density per volume can be increased by increasing the negative electrode density as mentioned above. However, the application of excessive pressure as high as 1.7 g/cm3 or more to a negative electrode in order to increase the density thereof may cause many problems, such as peeling of graphite in the negative electrode from a current collector and deterioration of charge-discharge properties due to the high crystalline anisotropy of graphite.
Spherical natural graphite is characterized in that it has good peel strength and thus is hardly peeled off from a current collector even when an electrode is pressed with a strong force. However, spherical natural graphite has high reaction activity with an electrolytic solution and low permeability to an electrolytic solution. Therefore, first cycle charge-discharge efficiency and rapid charge-discharge efficiency are still scope for improvement.